In a laser printer or a copier, an image (latent image) is made or produced by reflecting light that has been modulated using image data by a polygon mirror so as to scan a photoreceptor drum. The latent image is then developed using toner and the developed image is transferred to a print sheet. By doing so, a desired image can be printed.
Japanese Laid-Open Patent Publication No. H09-174917 discloses a technology where light that has been reflected by a polygon mirror moves at a constant speed in a main scanning direction via an fθ lens. The publication discloses that when doing so, errors relating to maintaining a constant speed caused by errors in the surface precision of the polygon mirror, errors relating to constant speed for the fθ lens, alignment errors for an optical unit, and the like are corrected by a combination of a short-period clock and a long-period clock using constant-speed correction data. This technology is aimed at preventing displacements in print positions from occurring due to errors in maintaining a constant speed.